By way of example and for the purpose of illustration, the invention will be described herein in connection with solvent-extracted soybean oilseed meal.
In U.S. Pat. No. 2,585,793, there is set forth a process in which solvent-extracted meal containing solvent or other liquid such as water is heated below the boiling-point of water, steam being introduced into the meal to condense on the meal and bring the moisture content of the meal into the range of about 12 to 30% by weight. Thereafter the meal is cooked at a temperature usually above the boiling-point of the water to produce a so-called toasted meal.
In the practice of the known process, the solvent-saturated meal or flakes are withdrawn from the extractor and passed through jacketed agitator equipment serving as a desolventizing means.
The solvent vapors are withdrawn and the meal or flakes substantially free of solvent are passed for further treatment.
In such a process, it was found that, at best, only a small amount of the steam employed could be introduced into lower compartments of the desolventizer, because, when a large volume of steam was employed in the lower compartments, flakes of the meal were carried from the topmost compartment together with the solvent vapors and some steam into the solvent vapor condenser.
It has further been suggested to utilize a so-called pre-desolventizing screw or other device, sometimes called a "SCHNECKEN" or PDS, which is placed between the solvent extractor (SE) and the desolventizer unit (DS) to pre-evaporate a certain percentage of solvent adhering on the meal before conveying the meal into the desolventizer. Such a pre-desolventizer is normally operated by indirect steam either with the aid of steam-heated plates or a steam-jacket forming the walls of the apparatus. The solvent adhering on the meal is partly vaporized and vented from the upper portion of the pre-desolventizer to a condenser via a vapor-washer to prevent meal-flakes entering the condenser.
Though a large portion of the solvent adhering on the meal is rapidly and positively eliminated in the PDS, there is considerable waste of the heat energy employed to vaporize a portion of the solvent in the pre-desolventizing station. Part of this energy leaves the PDS as the heat of vaporization in the solvent vapor discharged. Part of this energy is lost as a result of the superheat of these vapors. The temperature of the solvent vapors is far above the boiling temperature and ranges between about 85 and 100 degrees C. as compared to the nominal boiling range of the solvent (often hexane or its azeotrope, for example) of about 60 to 69 degrees C. at atmospheric pressure.
Furthermore, there is never a dust-free solvent vapor suitable for being recovered. This makes it necessary to install voluminous equipment downflow from the pre-desolventizer, and often downflow from the desolventizer, for washing and filtering the solvent vapors exiting these equipments. As a result, the size and amounts of equipment of the plant are significantly increased, as are the costs for auxiliary energy and maintenance.